technique for accommodating lower instep height than is ideal for a last and pattern of a lace-up shoe by lasting an upper a half size or more smaller than would usually be used on that size last
Depending on how the pattern is graded, using a smaller-sized upper can widen the facing gap over the foot, allowing the wearer to lace the shoe tight enough to hold the foot in place.
Compare oversizing uppers.
Mismatching last and upper sizes usually isn’t the ideal way to address instep height fit, but can be a relatively straightforward way for made-to-order production lines that don’t modify lasts to accommodate small differences.